With respect to photographic images made up of silver grains, the ratio of optical density of the image to a per unit area amount of silver which constitutes the image is generally called covering power. This ratio is employed as a measure for estimating the optical efficiency of the silver making up the image. The covering power of a silver halide photographic light-sensitive layer is, in general, higher the smaller the size of the silver halide grains, and decreases with increasing grain size. On the other hand, the photographic speed of a silver halide emulsion layer generally increases the larger the size of the silver halide grains. Consequently, silver halide emulsions having large grain sizes are employed for photographic materials having high photographic speeds. Accordingly, photographic light-sensitive materials having high photographic speeds require a large per unit area amount of silver for attaining some definite height of image density. Namely, if one intends the photographic material to have both high speed and required maximum image density, one must incorporate light-sensitive silver salts into the photographic material in a larger amount per unit area thereof. This is the real state of conventional photographic materials having high speeds.
One attempts at improving the covering power while maintaining high speed is a technique of adding various kinds of polymers to coarse-grained silver halide emulsions having high photographic speeds. This technique is described in British Pat. Nos. 1,048,057 and 1,039,471; and U.S. Pat. Nos. 3,043,697 and 3,446,618. The use of each of these polymers has some, though not a wholly satisfactory, effect on heightening the covering power. However, they lower the strength of coated films. Therefore, such a technique is unacceptable. This is because if photographic materials containing coated films having little strength are processed, especially using an automatic developing machine which is prevailingly employed at present, part of the gelatin contained therein is eluted with the developing solution or the fixing solution and adheres to the conveyor rollers of the automatic developing machine and further, is transferred onto photographic materials processed later, staining the photographic images produced thereon.
On the other hand, U.S. Pat. Nos. 2,996,382 and 3,178,282 describe that if silver halide photographic materials having a coarse-grained silver halide emulsion of the kind which forms latent images at the surface predominantly and a fine-grained silver halide emulsions having fogging nuclei inside the grains in the same layer or in the adjacent layers separately are employed, photographic images of high contrast and high covering power can be obtained at high speeds.
However, sufficiently high speed, density and contrast still cannot be acquired by using such a method. In addition, that method suffers from the disadvantages that it requires a long time for effecting development in the usual low temperature processing, whereas in the usual high temperature rapid processing it cannot exhibit the desired effects.
In order to improve the above-described disadvantages, a technique of incorporating a thioether compound (e.g. C.sub.6 H.sub.13 OCOCH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 OCOC.sub.6 H.sub.13) or a nitrogen-containing heterocyclic compound having a thioketone group into a photographic material having a light-sensitive silver halide emulsion and an internally fogged silver halide emulsion is proposed by published unexamined Japanese Patent Application 89749/82. However, satisfactory improvement upon the above-described disadvantages cannot be made with such a method.
Moreover, we have found previously (disclosed in Japanese Patent Application No. 22372/83) that such a compound as to be represented by the general formula (I) described hereinafter wherein both R.sub.1 and R.sub.2 are at least one hydroxyl group-containing alkyl, aryl or heterocyclic groups is effective for the improvement upon the above-described disadvantages, but its effect is insufficient. More specifically, it cannot contribute to acquirement of sufficiently high photographic speed, contrast and maximum density, especially when the bromine ion concentration in the developing solution is low.
On the other hand, we have also found and disclosed in Japanese Patent Application No. 210498/82 that the defects inherent in the technique described in U.S. Pat. Nos. 2,996,382 and 3,178,282; that is to say, a tendency to cause developmental fog and a tendency to cause uneven stains in a photographic material, can be lessened by the use of at least one mercapto group-containing tetrazaindenes, at least one mercapto group-containing purines, at least one mercapto group-containing triazaindenes or at least one mercapto group-containing pentazaindenes. However, all the capability of this technique cannot satisfy to the full the above-described requirements of a photographic material, that is, acquirements of sufficiently high photographic speed, contrast and maximum density. More specifically, with this technique desired effects cannot be produced to a satisfactory extent, especially under the condition of a low bromine ion concentration in the developing solution.
Since the bromine ion concentration in the developing solution varies with the number of processed films in the development using an automatic developing machine, which is usually employed on the market, and during development by hand, it is important to produce photographic images having unchanged properties, irrespective of the change of the bromine ion concentration.